W_Bis2A_Lecture02_reading - Biology LibreTexts
lakoid membranes into grana and stroma lamellae shows the redistribution possibilities for the relationship between grana structure and protein move- project was purchased with Wellcome Trust and BBSRC grants to. What is the difference between Grana and Stroma? Light reaction of photosynthesis occurs in grana. Dark reaction of the photosynthesis occurs. However, the relationship between LHCII phosphorylation and regulation of the LET/CET is increased, creating a larger contact area between grana and stromal lamellae. LEVERHULME TRUST (THE), RPG
It will take a lot longer for the glass of corn syrup to change color. The relevance of these examples is to note that the cytoplasm tends to be very viscous. It contains many proteins, metabolites, small molecules, etc. So, diffusion in cells is slower and more limited than you might have originally expected. Is there a potential problem to getting big that is related to the process of diffusion?
So how do cells get bigger? As you've likely concluded from the discussion above, with cells that rely on diffusion to move things around the cell—like bacteria and archaea—size does matter. Take a look at these links, and see what these bacteria look like morphologically and structurally: Based on what we have just discussed, in order for cells to get bigger, that is, for their volume to increase, intracellular transport must somehow become independent of diffusion.
One of the great evolutionary leaps was the ability of cells eukaryotic cells to transport compounds and materials intracellularly, independent of diffusion. Compartmentalization also provided a way to localize processes to smaller organelles, which overcame another problem caused by the large size. Compartmentalization and the complex intracellular transport systems have allowed eukaryotic cells to become very large in comparison to the diffusion-limited bacterial and archaeal cells.
We'll discuss specific solutions to these challenges in the following sections. Structure and Function Introduction to eukaryotic cells By definition, eukaryotic cells are cells that contain a membrane-bound nucleus, a structural feature that is not present in bacterial or archaeal cells. In addition to the nucleus, eukaryotic cells are characterized by numerous membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria, and others.
In previous sections, we began to consider the Design Challenge of making cells larger than a small bacterium—more precisely, growing cells to sizes at which, in the eyes of natural selection, relying on diffusion of substances for transport through a highly viscous cytosol comes with inherent functional trade-offs that offset most selective benefits of getting larger. In the lectures and readings on bacterial cell structure, we discovered some morphological features of large bacteria that allow them to effectively overcome diffusion-limited size barriers e.
As we transition our focus to eukaryotic cells, we want you to approach the study by constantly returning to the Design Challenge. This memorization exercise is necessary but not sufficient. Your instructors will, of course, propose some functional hypotheses for you to consider that address these broader points. Try using the Design Challenge rubric to explore some of your ideas.
These figures show the major organelles and other cell components of a a typical animal cell and b a typical eukaryotic plant cell. The plant cell has a cell wall, chloroplasts, plastids, and a central vacuole—structures not found in animal cells.
Plant cells do not have lysosomes or centrosomes. The plasma membrane Like bacteria and archaea, eukaryotic cells have a plasma membranea phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes such as carbon dioxide and ammonia also leave the cell by passing through the plasma membrane, usually with some help of protein transporters.
The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it. As discussed in the context of bacterial cell membranes, the plasma membranes of eukaryotic cells may also adopt unique conformations. The "folding" of the membrane into microvilli effectively increases the surface area for absorption while minimally impacting the cytosolic volume. Such cells can be found lining the small intestine, the organ that absorbs nutrients from digested food.
People with celiac disease have an immune response to gluten, a protein found in wheat, barley, and rye. The immune response damages microvilli. As a consequence, afflicted individuals have an impaired ability to absorb nutrients. This can lead to malnutrition, cramping, and diarrhea. Microvilli, shown here as they appear on cells lining the small intestine, increase the surface area available for absorption.
These microvilli are only found on the area of the plasma membrane that faces the cavity from which substances will be absorbed. It is composed of organelles suspended in the gel-like cytosolthe cytoskeleton, and various chemicals see figure below. Even though the cytoplasm consists of 70 to 80 percent water, it nevertheless has a semisolid consistency.
It is crowded in there.
Proteins, simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, ions and many other water-soluble molecules are all competing for space and water. The nucleus Typically, the nucleus is the most prominent organelle in a cell see figure below when viewed through a microscope.
The nucleus stores chromatin DNA plus proteins in a gel-like substance called the nucleoplasm. The nucleolus is a condensed region of chromatin where ribosome synthesis occurs. The boundary of the nucleus is called the nuclear envelope. It consists of two phospholipid bilayers: The nuclear membrane is continuous with the endoplasmic reticulum.
Nuclear pores allow substances to enter and exit the nucleus. The nuclear envelope The nuclear envelopea structure that constitutes the outermost boundary of the nucleus, is a double-membrane—both the inner and outer membranes of the nuclear envelope are phospholipid bilayers. The nuclear envelope is also punctuated with protein-based pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm.
Chromatin and chromosomes To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures within the nucleus that are made up of DNA, the hereditary material. You may remember that in bacteria and archaea, DNA is typically organized into one or more circular chromosome s.
In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific number of chromosomes in the nuclei of its cells.
Stroma (fluid) - Wikipedia
In humans, for example, the chromosome number is 23, while in fruit flies, it is 4. Chromosomes are only clearly visible and distinguishable from one another by visible optical microscopy when the cell is preparing to divide and the DNA is tightly packed by proteins into easily distinguishable shapes.
The term chromatin is used to describe chromosomes the protein-DNA complexes when they are both condensed and decondensed. Use the Design Challenge rubric to consider the nucleus in more detail. What "problems" does an organelle like the nucleus solve?
What are some of the qualities of a nucleus that may be responsible for ensuring its evolutionary success? What are some of the trade-offs of evolving and maintaining a nucleus? Every benefit has some cost; can you list both? Remember, there may be some well-established hypotheses and it is good to mention thesebut the point of the exercise here is for you to think critically and to critically discuss these ideas using your collective "smarts".
Ribosomes Ribosomes are the cellular structures responsible for protein synthesis. When viewed through an electron microscope, ribosomes appear either as clusters polyribosomes or single, tiny dots that float freely in the cytoplasm. They may be attached to the cytoplasmic side of the plasma membrane or the cytoplasmic side of the endoplasmic reticulum and the outer membrane of the nuclear envelope cartoon of cell above. Electron microscopy has shown us that ribosomes, which are large complexes of protein and RNA, consist of two subunits, aptly called large and small figure below.
The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein.
This is covered in greater detail in the section covering the process of translation. Ribosomes are made up of a large subunit top and a small subunit bottom. During protein synthesis, ribosomes assemble amino acids into proteins. Depending on the species and the type of mitochondria found in those cells, the respiratory pathways may be anaerobic or aerobic. By definition, when respiration is aerobic, the terminal electron is oxygen; when respiration is anaerobic, a compound other than oxygen functions as the terminal electron acceptor.
Nearly all mitochondria also possess a small genome that encodes genes whose functions are typically restricted to the mitochondrion. It is for instance possible muscle cells that are used—that by extension have a higher demand for ATP—may often be found to have a significantly higher number of mitochondria than cells that do not have a high energy load. The structure of the mitochondria can vary significantly depending on the organism and the state of the cell cycle which one is observing.
Both the inner and outer membranes are phospholipid bilayers embedded with proteins that mediate transport across them and catalyze various other biochemical reactions.
Difference Between Grana and Stroma
The inner membrane layer has folds called cristae that increase the surface area into which respiratory chain proteins can be embedded. The region within the cristae is called the mitochondrial matrix and contains—among other things—enzymes of the TCA cycle. During respiration, protons are pumped by respiratory chain complexes from the matrix into a region known as the intermembrane space between the inner and outer membranes.
This electron micrograph shows a mitochondrion as viewed with a transmission electron microscope. This organelle has an outer membrane and an inner membrane.
The inner membrane contains folds, called cristae, which increase its surface area. The space between the two membranes is called the intermembrane space, and the space inside the inner membrane is called the mitochondrial matrix. ATP synthesis takes place on the inner membrane. What are some of the functional challenges associated with coordinating processes that have a common set of molecules if the enzymes are sequestered into different cellular compartments? Peroxisomes Peroxisomes are small, round organelles enclosed by single membranes.
These organelles carry out redox reactions that oxidize and break down fatty acids and amino acids. They also help to detoxify many toxins that may enter the body. For example, alcohol is detoxified by peroxisomes in liver cells. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars.
Vesicles and vacuoles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them: Additionally, some agents such as enzymes within plant vacuoles break down macromolecules. The membrane of a vacuole does not fuse with the membranes of other cellular components. Animal cells versus plant cells At this point, you know that each eukaryotic cell has a plasma membrane, cytoplasm, a nucleus, ribosomes, mitochondria, peroxisomes, and in some, vacuoles.
There are some striking differences between animal and plant cells worth noting. Here is a brief list of differences that we want you to be familiar with and a slightly expanded description below: While all eukaryotic cells use microtubule and motor protein the based mechanisms to segregate chromosomes during cell division, the structures used to organize these microtubules differ in plants versus animal and yeast cells.
Animal and yeast cells organize and anchor their microtubules into structures called microtubule organizing centers MTOCs. Two centrioles organize into a structure called a centrosome. By contrast, in plants, while microtubules also organize into discrete bundles, there are no conspicuous structures similar to the MTOCs seen in animal and yeast cells.
Rather, depending on the organism, it appears that there can be several places where these bundles of microtubules can nucleate from places called acentriolar without centriole microtubule organizing centers. Animal cells typically have organelles called lysosomes responsible for degradation of biomolecules. Some plant biologists call these organelles lysosomes while others lump them into the general category of plastids and do not give them a specific name.
Plant cells have a cell wall, chloroplasts and other specialized plastids, and a large central vacuole, whereas animal cells do not. The centrosome The centrosome is a microtubule-organizing center found near the nuclei of animal cells. It contains a pair of centrioles, two structures that lie perpendicular to eachother see figure below. Each centriole is a cylinder of nine triplets of microtubules. The centrosome consists of two centrioles that lie at right angles to each other.
Each centriole is a cylinder made up of nine triplets of microtubules.
Difference Between Grana and Stroma | Grana vs Stroma
Nontubulin proteins indicated by the green lines hold the microtubule triplets together. The centrosome the organelle where all microtubules originate in animal and yeast replicates itself before a cell divides, and the centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell.
Lysosomes Animal cells have another set of organelles not found in plant cells: Enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and even "worn-out" organelles.
These enzymes are active at a much lower pH than that of the cytoplasm. Therefore, the pH within lysosomes is more acidic than the pH of the cytoplasm. In plant cells, many of the same digestive processes take place in vacuoles. Fungal and protistan cells also have cell walls. While the chief component of bacterial cell walls is peptidoglycan, the major organic molecule in the plant cell wall is cellulose see structure belowa polysaccharide made up of glucose subunits.
The dashed lines at each end of the figure indicate a series of many more glucose units. The size of the page makes it impossible to portray an entire cellulose molecule. Chloroplasts Chloroplasts are plant cell organelles that carry out photosynthesis. Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. The fluid enclosed by the inner membrane that surrounds the grana is called the stroma.
The chloroplast has an outer membrane, an inner membrane, and membrane structures called thylakoids that are stacked into grana. The space inside the thylakoid membranes is called the thylakoid space. The light harvesting reactions take place in the thylakoid membranes, and the synthesis of sugar takes place in the fluid inside the inner membrane, which is called the stroma. Chloroplasts also have their own genome, which is contained on a single circular chromosome.
The chloroplasts contain a green pigment called chlorophyll, which captures the light energy that drives the reactions of photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria perform photosynthesis, but their chlorophyll is not relegated to an organelle. Evolution connection Endosymbiosis We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes.
Have you wondered why? Strong evidence points to endosymbiosis as the explanation. Symbiosis is a relationship in which organisms from two separate species depend on each other for their survival.
Endosymbiotic relationships abound in nature. The relationship between these microbes and us their hosts is said to be mutually beneficial or symbiotic. The relationship is beneficial for us because we are unable to synthesize vitamin K; the microbes do it for us instead. Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that bacteria have DNA and ribosomes, just as mitochondria and chloroplasts do.
Scientists believe that host cells and bacteria formed an endosymbiotic relationship when the host cells ingested both aerobic and autotrophic bacteria cyanobacteria but did not destroy them. Through many millions of years of evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the autotrophic bacteria becoming chloroplasts.
There will be more on this later in the reading. The central vacuole Previously, we mentioned vacuoles as essential components of plant cells. If you look at the cartoon figure of the plant cell, you will see that it depicts a large central vacuole that occupies most of the area of the cell.
Have you ever noticed that if you forget to water a plant for a few days, it wilts? As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant. The central vacuole also supports the expansion of the cell. When the central vacuole holds more water, the cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm. The scientific method overview An example of oversimplification that confounds many students of biology particularly early in their studies is the use of language that hides the experimental process used to build knowledge.
For the sake of expediency, we often tell stories about biological systems as if we are presenting unquestionable facts. However, while we often write and speak about topics in biology with a conviction that gives the appearance of "factual" knowledge, reality is often more nuanced and filled with significant uncertainties.
The "factual" presentation of material usually lacking discussion of evidence or confidence in the evidence plays to our natural tendency to feel good about "knowing" things, but it tends to create a false sense of security in the state of knowledge and does little to encourage the use of imagination or the development of critical thinking. Unfortunately, repeated qualification becomes rather cumbersome. The important thing to remember is that while we may not say so explicitly, all of the knowledge we discuss in class represents only the best of our current understanding.
Stromal thylakoids are also called intergranal thylakoids or lamellae. Both thylakoid and stromal thylakoid contain photosynthetic pigments on their surfaces.
On that account, the light reaction of photosynthesis occurs on the surface of grana. A granum is shown in figure 1. Granum Thylakoid is a round pillow-shaped stack inside the chloroplast. The space between thylakoid membrane is called thylakoid lumen. Chlorophyll and other photosynthetic pigments are held by membrane proteins on the surface of the thylakoid. They are organized into photosystem 1 and 2 on the thylakoid membrane.
- Stroma (fluid)
What is Stroma Stroma refers to a colorless jell-like matrix of the chloroplast in which the dark reaction of photosynthesis takes place. Enzymes required for the dark reaction are embedded in the stroma. Stroma surrounds the grana.
In the stroma, carbon dioxide and water are used in the production of simple carbohydrates by using the light energy trapped by light reaction.
Stroma and grana of a chloroplast are shown in figure 2. Structure of a Chloroplast Dark reaction of photosynthesis is also called the Calvin cycle.
The three stages of the Calvin cycle are carbon fixation, reduction reactions, and RuBP regeneration. Similarities Between Grana and Stroma Both grana and stroma are two structures of the chloroplast. Reactions of photosynthesis occur in both grana and stroma.